1// Copyright 2018 The Abseil Authors.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// https://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14//
15// https://code.google.com/p/cityhash/
16//
17// This file provides a few functions for hashing strings. All of them are
18// high-quality functions in the sense that they pass standard tests such
19// as Austin Appleby's SMHasher. They are also fast.
20//
21// For 64-bit x86 code, on short strings, we don't know of anything faster than
22// CityHash64 that is of comparable quality. We believe our nearest competitor
23// is Murmur3. For 64-bit x86 code, CityHash64 is an excellent choice for hash
24// tables and most other hashing (excluding cryptography).
25//
26// For 32-bit x86 code, we don't know of anything faster than CityHash32 that
27// is of comparable quality. We believe our nearest competitor is Murmur3A.
28// (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
29//
30// Functions in the CityHash family are not suitable for cryptography.
31//
32// Please see CityHash's README file for more details on our performance
33// measurements and so on.
34//
35// WARNING: This code has been only lightly tested on big-endian platforms!
36// It is known to work well on little-endian platforms that have a small penalty
37// for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
38// It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
39// bug reports are welcome.
40//
41// By the way, for some hash functions, given strings a and b, the hash
42// of a+b is easily derived from the hashes of a and b. This property
43// doesn't hold for any hash functions in this file.
44
45#ifndef ABSL_HASH_INTERNAL_CITY_H_
46#define ABSL_HASH_INTERNAL_CITY_H_
47
48#include <stdint.h>
49#include <stdlib.h> // for size_t.
50#include <utility>
51
52namespace absl {
53namespace hash_internal {
54
55typedef std::pair<uint64_t, uint64_t> uint128;
56
57inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
58inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
59
60// Hash function for a byte array.
61uint64_t CityHash64(const char *s, size_t len);
62
63// Hash function for a byte array. For convenience, a 64-bit seed is also
64// hashed into the result.
65uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
66
67// Hash function for a byte array. For convenience, two seeds are also
68// hashed into the result.
69uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
70 uint64_t seed1);
71
72// Hash function for a byte array. Most useful in 32-bit binaries.
73uint32_t CityHash32(const char *s, size_t len);
74
75// Hash 128 input bits down to 64 bits of output.
76// This is intended to be a reasonably good hash function.
77inline uint64_t Hash128to64(const uint128 &x) {
78 // Murmur-inspired hashing.
79 const uint64_t kMul = 0x9ddfea08eb382d69ULL;
80 uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
81 a ^= (a >> 47);
82 uint64_t b = (Uint128High64(x) ^ a) * kMul;
83 b ^= (b >> 47);
84 b *= kMul;
85 return b;
86}
87
88} // namespace hash_internal
89} // namespace absl
90
91#endif // ABSL_HASH_INTERNAL_CITY_H_
92